Refer to FIG. 1. Most of power supply devices (including AC/DC and DC/DC converters) comprise: a rectifier filter unit 1, a booster unit 2 (including the conventional power factor correction units), and a switching type power conversion unit 3. An input power 8 is received by the power supply device from a power source, processed by the rectifier filter unit 1 and boosted by the booster unit 2 (including the conventional power factor correction units) to form a boosted power. Then, the boosted power is converted into an output power 9 by the power conversion unit 3 to drive at least one load. The booster unit 2 further comprises: a charge/discharge loop, a switch element, and a boost control circuit, wherein the boost control circuit can generate a driving signal to turn on the switch element. The boost control circuit adjusts the duty ratio (i.e. the width of the turn-on period of the driving signal) to control the turn-on interval of the switch element and then control the voltage of the charge/discharge loop. Thereby, the input power 8 is converted into the boosted power. In addition to adjusting the duty ratio, the conventional booster unit 2 may have a Continuous Current Mode (CCM) or a Discontinuous Current Mode (DCM). DCM is usually only used in small power supply devices, but CCM dominates power supply devices. Constrained by its working principle, CCM is a fixed-frequency method. In other words, the booster unit 2 uses a fixed-frequency driving signal to control the switch element and then control the current cycles of the charge/discharge loop, wherein the duty ratio is influenced by the load. However, the turn-on interval of the switch element must vary with the input current to a considerable extent so that the booster unit 2 (including the conventional power factor correction units) can provide a constant-voltage boosted power. For example, the voltage of the input power 8 may vary from 90V to 150V; thus, the turn-on interval of the switch element should be greatly prolonged to attain a boosted voltage of 290V so that the booster unit 2 can output a boosted voltage of 380V. Consequently, the switch element has to withstand very great current, and the switching loss is also very obvious. Besides, the influences of the harmonics and noise are also very obvious. Therefore, the conventional technology has much room to improve.